Analogous to the electronic diode, a thermal diode transports heat mainly in one preferential direction rather than in the opposite direction. Phase change thermal diodes usually rectify heat transport much more effectively than solid state thermal diodes due to the latent heat phase change effect. However, they are limited by either the gravitational orientation or one dimensional configuration. On the other hand, solid state thermal diodes come in many shapes and sizes, durable, relatively easy to construct, and are simple to operate, but their diodicity (rectification coefficient) is always in the order of η˜1 or lower, which is too small for practical applications. In order to be practically useful for most engineering systems, a thermal diode should exhibit a diodicity in the order of η˜10 or greater.
The effectiveness of a thermal diode is measured by the rectification coefficient (diodicity) which is given by,
                    η        =                                            k              f                        -                          k              r                                            k            r                                              (        1        )                            where kf and kr are the effective thermal conductivities in the forward and reverse operating modes, respectively. When heat transfers in the preferential direction with high conductance, the thermal diode is operating in forward mode. When heat transfers in the opposite direction with low conductance, the thermal diode is operating in reverse mode. To maximize the diodicity, the heat transfer in the forward mode should be maximized, while the heat transfer in the reverse mode should be prevented.        
The thermal diode of the present embodiments includes a heat source, a heat sink and a thermal coupling element, which are all metal blocks (i.e. copper, aluminum, and iron). In the forward mode, the thermal coupling element is in contact with the heat source and heat sink. Since metal is a good thermal conductor, a good heat transfer occurs in the forward mode. In the reverse mode, the thermal coupling element is moved out of the thermal contact with the heat source and heat sink. Since air is a good thermal insulator, heat transfer is effectively prevented in the reverse mode.
An electrical motor is a good device for controlling the movement of the metal blocks. However, it requires electrical energy.
Therefore, it is desirable to develop a passive solid thermal diode with a large diodicity.